Monmouth, OR has a population of 9,919 and is the 3,957th largest city in the United States. The population density is 4,572 per sq mi which is 11446% higher than the Oregon average and 4947% higher than the national average. The median age in Monmouth is 23 which is approximately 40% lower than the Oregon average of 39. In Monmouth, 37% of the population over 15 years of age are married, 81% speak English and 12% speak Spanish. 53% of Monmouth residents were born in Oregon, 36% were born out of state, 0% were born outside of the United States and 11% were foreign born.